Well, ignoring that you've pointed to an example of a licensed cab driver, as that's "not the point being made here", there really isn't a point to be made.

What you've pointed to is a risk of allowing people to interact, and really has very little (if any) to do with cab regulation. Being murdered is a risk anytime you're around other people; its just so remote that usually it is appropriately ignored.

Further, if there were an epidemic of cab-murders, which of course a single anecdote doesn't show, such that it made sense to worry about the risk of homicide, then the people calling cabs would be aware of this, and there would be market demand for a cab company that does full background checks, offers life-insurance policies, sends a second person to guard against malfeasance, etc. Of course, murder is illegal, so police would be looking for the perpetrators as well.

Even if for some reason a that didn't happen (say, people enjoy being murdered during their cab ride) and we wanted to prevent it anyway, the type of regulations that'd make sense would be things like background checks before issuing a license (which would need renewing routinely), which would apply to all cab operators, both new and renewing.

Keep in mind that the regulations that people actually object to are mostly of the form "we allow X cabs total in the city".